DE 10 2012 024 406 A1 discloses a method of the type in question for producing a piston, in which at least the circumferential piston-skirt region close to the piston head is composed of a light metal alloy which is suitable for forging and has at least one ring carrier to receive a piston ring. The known method starts with a piston having a stepped circumferential surface, more specifically such that, starting from a step, the piston has a reduced diameter toward the piston head and that the ring carrier is placed on this step and then connected firmly to the piston by forging the piston. However, the disadvantage with this method is that relatively high forces act during the forging in of the ring carrier, and these must be dissipated via the previously forged and relatively thin-walled internal shape region, leading there to high loads and possibly also causing damage associated with said loads.
DE 33 00 582 C2 discloses a method involving powder metallurgy for the production of ring carriers on pistons composed of austenitic iron alloys. In this case, a charge of the iron alloy is first of all melted in a furnace, the melt is poured out and atomized by means of a water, air or gas stream to produce powder with grain sizes in a range of from 0.044 to 0.42 mm with an austenitic white cast iron structure and no effective green strength. The material produced in this way is then annealed in a reducing atmosphere and a quantity of lubricant sufficient to ensure that subsequent compression to give the desired groove shape results in a maximum possible green density is added to said material, and then the lubricant is burnt off in a protective atmosphere, after which the compressed material is sintered and then abruptly cooled.
In general, increasing ignition pressures and higher combustion temperatures are a known means in engine development of enabling fuel consumption to be reduced. However, increased ignition pressures and higher combustion temperatures also make greater demands on the materials used for the pistons, and therefore these are usually produced from aluminum alloys in a gravity die casting process. For special requirements, forged pistons are also produced for spark ignition engines since a forming process may give them better physical material characteristics in some circumstances.
Also on account of the increasing ignition pressures, ring carriers for reinforcing a first piston ring groove are also increasingly being cast into the piston, wherein these ring carriers generally produced from an austenitic cast iron by centrifugal casting and by permanent mold casting using an Al-Fin process, which brings about a metallurgical bond between the ring carrier and the piston alloy. However, an Al-Fin process of this kind cannot be applied to forged ring-carrier pistons since it is not possible for a metallurgical bond to develop owing to the existing oxide skin on the surface of the forging blank.
In the case of forged pistons, therefore, the practice hitherto has been, for example, to machine a larger groove than the subsequent groove in the production piston into a forging blank with contours close to the final state and then to fill this groove with a wear-resistant material by a thermal coating method. In further machining operations, the final geometry of the annular groove was then produced, wherein the groove reinforcement is ensured by the wear-resistant material applied by thermal spraying. However, a method of this kind is relatively complex and, as a result, extremely expensive.
Another possibility is provided by the forging in of the ring carrier, a method which is known from DE 10 2012 024 406 A1, for example, wherein, in a first forming step, the upset piston blank is initially preformed or preprocessed in such a way that a surface on which the ring carrier can be positioned before the second forming step is preformed at the ultimate position of the piston groove reinforcement. However, it was only possible to produce mechanical interlocking between the ring carrier and the piston blank in the second forming step if the ring carrier had been provided with undercuts, e.g. by mechanical processing, before being forged in. However, the disadvantage with this method are the relatively high production costs, which are caused, in particular, by the production of the undercuts and the necessity of an additional forging or processing step.
It has furthermore been found that, owing to the differing expansion coefficients of the aluminum alloys of the piston materials and of the iron alloys of the potential ring carrier alloy, the mechanically produced undercuts are not sufficient at high ignition pressures to permanently ensure a mechanical bond and avoid damage due to the failure of the ring carrier bond with the piston.